Data Link Protocols. 11 장 Data Link Control and Protocols. 11.1 Flow and Error Control 11.2 Stop-and-Wait ARQ 11.3 Go-Back-N-ARQ 11.4 Selective Repeat ARQ 11.5 HDLC. Flow Control.
11.1 Flow and Error Control
11.2 Stop-and-Wait ARQ
11.4 Selective Repeat ARQ
of the frame received in one single frame
In a Stop-and-Wait ARQ system, the bandwidth of the line is 1 Mbps, and 1 bit takes 20 ms to make a round trip. What is the bandwidth-delay product? If the system data frames are 1000 bits in length, what is the utilization percentage of the link?
The bandwidth-delay product is a measure of the number of bits
a system can have in transit.
The bandwidth-delay product is
1 106 20 10-3 = 20,000 bits
The system can send 20,000 bits during the time it takes for the data to go from the sender to the receiver and then back again. However, the system sends only 1000 bits. We can say that the link utilization is only 1000/20,000, or 5%. For this reason, for a link with high bandwidth or long delay, use of Stop-and-Wait ARQ wastes the capacity of the link.
What is the utilization percentage of the link in Example 1 if the link uses Go-Back-N ARQ with a 15-frame sequence?
The bandwidth-delay product is still 20,000. The system can send up to 15 frames or 15,000 bits during a round trip. This means the utilization is 15,000/20,000, or 75 percent. Of course, if there are damaged frames, the utilization percentage is much less because frames have to be resent.
HDLC is a protocol that implements ARQ mechanisms.
It supports communication over point-to-point or multipoint links.
~ contains the address of the secondary station that is either the originator or destination of the frame
~ is used for acknowledgment, flow control, and error control
Set normal response mode
Set normal response mode(extended)
Set asynchronous response mode
Set asynchronous response mode(extended)
Set asynchronous balanced mode
Set asynchronous balanced mode(extended)
Request information mode
Set initialization mode
~ can be divided into five basic functional category
Figure 11.22 shows an exchange using piggybacking where is no error. Station A begins the exchange of information with an I-frame numbered 0 followed by another I-frame numbered 1. Station B piggybacks its acknowledgment of both frames onto an I-frame of its own. Station B’s first I-frame is also numbered 0 [N(S) field] and contains a 2 in its N(R) field, acknowledging the receipt of A’s frames 1 and 0 and indicating that it expects frame 2 to arrive next. Station B transmits its second and third I-frames (numbered 1 and 2) before accepting further frames from station A. Its N(R) information, therefore, has not changed: B frames 1 and 2 indicate that station B is still expecting A frame 2 to arrive next.
In Example 3, suppose frame 1 sent from station B to station A has an error. Station A informs station B to resend frames 1 and 2 (the system is using the Go-Back-N mechanism). Station A sends a reject supervisory frame to announce the error in frame 1. Figure 11.23 shows the exchange.
Bit stuffing is the process of adding one extra 0 whenever there are five consecutive 1s in the data so that the receiver does not mistake the data for a flag.
~ provides those basic control function required for communication between a DTE and a DCE
~ is used only in balanced configuration of two devices
~ is used in ISDN on B channels
~ used in ISDN
~ use ABM(Asynchronous Balanced Mode)
~ is designed to do asynchronous-synchronous conversation, error detection, and retransmission
~ has become developed to apply HDLC feature to modem